1. Field of the Invention
The present invention relates to a cooking device or unit with a glass-ceramic panel providing a cooking surface, which is not transparent and which has at least one decoration, on it. The invention also relates to a method of making this glass-ceramic panel.
2. Description of the Related Art
Cooking units or devices for modern kitchens including glass-ceramic panels providing cooking surfaces have been known for years and have proven completely satisfactory to customers. They are heated by a variety of different heating devices, including electrically heated radiant heating bodies, atmospheric gas burners, halogen radiators, gas heated burner units and induction heating devices.
These glass ceramic panels have a series of requirements, apart from their heating specifications. Especially the heating and operating parts under the glass ceramic panel in the cooking unit should not be observable from above the glass ceramic panel. Marks or streaks made during usage should not be conspicuous. Decorations, should be applied for aesthetic reasons and to demark functional areas, such as cooking zones, from other regions with different appealing colors.
The prior art provides a series of solutions for these requirements, which however have specific disadvantages.
In a typical known solution, like that described in EP 0 220 333, a colored glass-ceramic panel with high quartz mixed crystals as the predominant crystalline phase is employed. These glass-ceramic panels acting as cooking surfaces have been marketed for a long time, for example under the trademark CERAN® by Schott Glas. These cooking surfaces appear dark to black to the eye and thus prevent observation of the cooking unit interior.
The very smooth, lustrous aesthetically pleasing surfaces of the glass ceramic panels have certain susceptibilities for conspicuous marks or scratches on their surfaces that are caused by pots, metal abrasion and scrapes. Also fingerprints are conspicuous and noticeable. There is always an effort made of course to reduce the conspicuousness of marks on the glass ceramic panel by means of some type of decoration, for example an impervious decoration or even a full-surface decoration.
Thus EP 0 693 464 B1 describes a glass ceramic panel that is darkly colored and decorated on its upper surface, i.e. the cooking surface, with ceramic paints or pigments, on which at least one additional colored coating is arranged over a first colored coating, which at least partially overlaps the first colored coating. This first colored coating acts as a base decoration that coats a large area of the surface of the glass ceramic panel to form a uniform scratch-protecting layer. The other different colored coatings are marking decorations, e.g. for cooking zones, and simplify the confusion-free identification of the operating regions of the glass ceramic panel by means of the different colors.
Both decorations or colored coatings may overlap partially only in edge regions, because otherwise the colored coatings have difficulty adhering on each other, i.e. they have adherence problems. That is because of the different thermal expansion coefficients of the glass ceramic panel (which varies between 0±0.3×10−6/K from 20 to 700° C.) and of the decorations, i.e. colored coatings (>about 5×10−6/K). Only thin colored coatings having a thickness of up to about 7 μm allow the dark background of the glass ceramic panel to tube covered. Coatings with a larger thickness crack or split because of the differing thermal expansion coefficients, and/or poor adherence of the decorative coatings on these regions. Generally the possible colored embodiments are limited in the decoration of “black” glass ceramic panels because of the limited color coating thickness. The decorative paint produces only pale, less intense color shades and preferably only dark color shades. A bright decoration print is achievable by a full-surface printing only to a limited extent. Each screen-printing fault, local variation or removal of the bright decorative paint in usage is exceptionally conspicuous because of the underlying black glass-ceramic panel.
A coating for solution of these problems in the manufacture of a full-surface decorative coating with ceramic paints on a cooking surface provided by a darkly colored glass ceramic panel is described in German Patent Document DE 197 28 881 C1. This coating is made by applying at least two grid-like or raster-like structural patterns corresponding to each other and in a side-by-side relationship on the glass ceramic panel to form a closed full-surface covering coating. In order to obtain a full-surface completely covering coating the respective screens used in the printing process must fit exactly together (positive and negative structures). Even so, an arduous positioning must take place in this printing process. Two printing processes are required in order to obtain a full-surface completely covered appearance covering the “black” glass ceramic panel. The possible embodiments are limited by overlap of the decorative paints because of differing thermal expansion coefficients.
In order to avoid limitations during application of the decorative coating due to the dark color of the glass ceramic panel, attempts have been made to use a colorless, i.e. transparent glass ceramic panel with high quartz mixed crystals as the predominant crystalline phase. The non-transparency of the glass ceramic panel required to prevent observation of the interior of the cooking unit from above is provided by a printed colored coating.
German Patent Document DE 200 05 461 U1 discloses a colorless transparent glass ceramic panel that provides a cooking surface and is printed on its underside with a single layer or multi-layer coating of temperature-resistant paint. This colored coating provides the non-transparency, i.e. it replaces the otherwise usual coloring of the panel, so that the panel appears dark from above. Also the bottom-side coating on the glass ceramic panel is formed as a color-imparting decoration. In contrast, the upper side of the glass ceramic panel is decoration-free, in as much as it is not coated. However dirt, stains and the like, e.g. fingerprints, are conspicuous to some extent on this known cooking unit and there is no protection of the top surface from superficial scratches, metal rub-off and marks due to usage because there is only a single coating on the underside of the glass ceramic panel.
German Patent Document DE 200 19 210.8 U1 describes a cooking unit with a colorless transparent glass ceramic panel that provides a cooking surface, which has a single colored coating on its underside and a full-surface decorative coating on its upper side. To prove an acceptable full-surface appearance a coating must be provided on both the top and bottom sides of the glass ceramic panel because of its transparency. Screen printing errors or variations of the upper side full-surface decorative coating are partially compensated by the bottom side single colored coating. Printing both sides however is very expensive and special requirements exist for the coating on the bottom side of the glass ceramic panel. Also the embodiments are limited by overlap of the decorative paints because of the differing thermal expansion coefficients.
Beside the described dark-colored glass ceramic panels or transparent glass ceramic panels provided with a dark colored coating, opaque white glass ceramic panels and translucent white glass ceramic panels providing cooking surfaces are known in the art. These latter glass ceramic panels have keatite mixed crystals as the predominant crystalline phase and are usually available with higher thermal expansion coefficients of about 0.8 to 1.5×10−6/K between 20 to 700° C. A multi-colored marking decoration is usually printed on the cooking surface side and burned in.
The phenomenon known as conspicuous staining, namely dirt, overflowing cooking materials and cleaning material residues necessarily collecting in scratches on the cooking surface during cooking, which are conspicuous on the white panel surface disadvantageously appear on these opaque white and translucent white glass ceramic panels.
An additional disadvantage is that the white glass ceramic panel becomes yellow colored in the vicinity of the heated cooking zone after shutting off the heating body. This coloring, designated thermochromism, is based on the thermal widening of the absorption band of TiO2, a required ingredient of the glass ceramic material.
The already described German Patent Document DE 200 19 210.8 U1 discloses a translucent bright glass ceramic panel. In this special embodiment the colored pigments of the full-surface decorative coating on the topside and the single color coating on the underside are selected, so that the glass ceramic panel appears white or creamy white or weakly colored from above. This creamy white or weakly colored coating in connection with the upper side full-surface decorative coating reduces the conspicuousness of dirt, stains and the like, especially fingerprints, in a special manner. The topside full-surface decorative coating provides further protection from upper surface scratches, metal rub off and usage marks. However the other disadvantages described above remain.
To avoid the above-described disadvantages it is also known to provide the glass ceramic panel with a bright appearance, especially a bright beige or creamy white color shade, designated “BISQUE” in the following, by suitably coloring the glass ceramic panel in the starting glass melt. German Patent Document DE 198 57 117 A1 describes cooking unit with a opaque uniformly colored glass ceramic panel with keatite mixed crystals as the predominant crystalline phase, in which the cooking surface of ceramicizable glass is ceramicized in a color range in the lab system with a lightness value L<85. The cooking surface has minimal dirt and stain conspicuousness in usage because of this coloring.
It is disadvantageous for coloring the glass ceramic material in bulk by pigment oxides that this coloring must occur in the melt. It is not economical for the manufacturer to make different color shades according to customer choices because of the size of the melt vessel, since the material obtained cannot be used for making a new color shade by re-melting. Also a costly storage unit is required for the various color tones. The desire of the customer for individual color shades and comparatively small lots cannot be satisfied economically. This flexibility is lost because of the large-scale coloring by pigment oxides in the melt. Paint or pigment feeders are not available currently for the high temperatures, which are required in the glass ceramic melts.